Resting heart rate and incident atrial fibrillation: A stratified Mendelian randomization in the AFGen consortium

Background

Both elevated and low resting heart rates are associated with atrial fibrillation (AF), suggesting a U-shaped relationship. However, evidence for a U-shaped causal association between genetically-determined resting heart rate and incident AF is limited. We investigated potential directional changes of the causal association between genetically-determined resting heart rate and incident AF.

Method and results

Seven cohorts of the AFGen consortium contributed data to this meta-analysis. All participants were of European ancestry with known AF status, genotype information, and a heart rate measurement from a baseline electrocardiogram (ECG). Three strata of instrumental variable-free resting heart rate were used to assess possible non-linear associations between genetically-determined resting heart rate and the logarithm of the incident AF hazard rate: <65; 65–75; and >75 beats per minute (bpm). Mendelian randomization analyses using a weighted resting heart rate polygenic risk score were performed for each stratum.

We studied 38,981 individuals (mean age 59±10 years, 54% women) with a mean resting heart rate of 67±11 bpm. During a mean follow-up of 13±5 years, 4,779 (12%) individuals developed AF. A U-shaped association between the resting heart rate and the incident AF-hazard ratio was observed. Genetically-determined resting heart rate was inversely associated with incident AF for instrumental variable-free resting heart rates below 65 bpm (hazard ratio for genetically-determined resting heart rate, 0.96; 95% confidence interval, 0.94–0.99; p = 0.01). Genetically-determined resting heart rate was not associated with incident AF in the other two strata.

Conclusions

For resting heart rates below 65 bpm, our results support an inverse causal association between genetically-determined resting heart rate and incident AF.

Common genetic variation near the connexin-43 gene is associated with resting heart rate in African Americans: a genome-wide association study of 13,372 participants

BACKGROUND

Genome-wide association studies have identified several genetic loci associated with variation in resting heart rate in European and Asian populations. No study has evaluated genetic variants associated with heart rate in African Americans.

OBJECTIVE

To identify novel genetic variants associated with resting heart rate in African Americans.

METHODS

Ten cohort studies participating in the Candidate-gene Association Resource and Continental Origins and Genetic Epidemiology Network consortia performed genome-wide genotyping of single nucleotide polymorphisms (SNPs) and imputed 2,954,965 SNPs using HapMap YRI and CEU panels in 13,372 participants of African ancestry. Each study measured the RR interval (ms) from 10-second resting 12-lead electrocardiograms and estimated RR-SNP associations using covariate-adjusted linear regression. Random-effects meta-analysis was used to combine cohort-specific measures of association and identify genome-wide significant loci (P≤2.5×10(-8)).

RESULTS

Fourteen SNPs on chromosome 6q22 exceeded the genome-wide significance threshold. The most significant association was for rs9320841 (+13 ms per minor allele; P = 4.98×10(-15)). This SNP was approximately 350 kb downstream of GJA1, a locus previously identified as harboring SNPs associated with heart rate in Europeans. Adjustment for rs9320841 also attenuated the association between the remaining 13 SNPs in this region and heart rate. In addition, SNPs in MYH6, which have been identified in European genome-wide association study, were associated with similar changes in the resting heart rate as this population of African Americans.

CONCLUSIONS

An intergenic region downstream of GJA1 (the gene encoding connexin 43, the major protein of the human myocardial gap junction) and an intragenic region within MYH6 are associated with variation in resting heart rate in African Americans as well as in populations of European and Asian origin.

Comparative performance of gene-based warfarin dosing algorithms in a multiethnic population

SUMMARY BACKGROUND

Gene-based warfarin dosing algorithms have largely been developed in homogeneous populations, and their generalizability has not been established.

OBJECTIVES

We sought to assess the performance of published algorithms in a racially diverse and multiethnic sample, and determine if additional clinical variables or genetic variants associated with dose could enhance algorithm performance.

PATIENTS AND METHODS

In 145 compliant patients on warfarin with a goal international normalized ratio (INR) of 2-3, stable, therapeutic doses were compared with predicted doses using 12 reported algorithms that incorporated CYP2C9 and VKORC1 variants. Additional covariates tested with each model included race, concurrent medications, medications known to interact with warfarin and previously described CYP4F2, CALU and GGCX variants.

RESULTS

The mean patient age was 67 +/- 14 years; 90 (62%) were male. Eighty-two (57%) were Caucasian, 28 (19%) African-American, 20 (14%) Hispanic and 15 (10%) Asian. The median warfarin dose was 35 mg per week (interquartile range 23-53 mg per week). Gene-based dosing algorithms explained 37-55% of the variation in warfarin dose requirements. Neither the addition of race, number of concurrent medications nor the number of concurrent medications interacting with warfarin enhanced algorithm performance. Similarly, consideration of CYP4F2, CALU or GGCX variant genotypes did not improve algorithms.

CONCLUSIONS

Existing gene-based dosing algorithms explained between approximately one-third and one-half of the variability in warfarin dose requirements in this racially and ethnically diverse cohort. Additional clinical and recently described genetic variants associated with warfarin dose did not enhance prediction in our patient population.

Improved Survival With Statins, Angiotensin Receptor Blockers, and Steroid Weaning After Heart Transplantation

Various immunosuppressive and adjunctive pharmacological regimens exist for cardiac transplantation, though the associations between these regimens and long-term survival are unclear. We reviewed demographic, clinical, and pharmacological data from 220 consecutive adult heart transplant recipients between 1986 and 2003 who survived beyond 3 months. Immunosuppression was cyclosporine-based (n=94) or tacrolimus-based (n=126), and 104 patients were weaned off steroids (all receiving tacrolimus). Covariates of mortality were assessed in a Cox proportional hazards analysis. The mean age was 5.2+/-13 years. Survival was 96%, 88%, and 81% at 1, 3, and 5 years, respectively. Significant covariates associated with mortality included pretransplant diabetes mellitus (hazard ratio [HR] 2.83, 95% confidence interval [CI] 1.45 to 5.04), black race (HR 1.41, 95% CI 1.01 to 1.94), higher pretransplant creatinine clearance (HR 0.99, 95% CI 0.98 to 1.00), steroid withdrawal (HR 0.60, 95% CI 0.39 to 0.85), and exposure to a statin (HR 0.53, 95% CI 0.40 to 0.70) or an angiotensin receptor blocker (HR 0.50, 95% CI 0.20 to 0.95) after transplantation. Treatment with a statin, an angiotensin receptor blocker, and steroid withdrawal were each associated with improved survival in heart transplant recipients. These findings warrant prospective study, with specific emphasis on identifying the clinical effects of these medications in transplant recipients.